Tortoise Resurrection

In a subsequent portion of this narrative I shall have frequent occasion to mention this species of tortoise. It is found principally, as most of my readers may know, in the group of islands known as the Gallipagos... They are frequently found of an enormous size... They can exist without food for an almost incredible length of time, instances having been known wher they have been thrown into the hold of a vessel and lain two years without nourishment of any kind - being as fat, and, in every respect, in as good order at the expiration of that time as when they were first put in... They are excellent and highly nutritious food, and have, no doubt, been the means of preserving the lives of thousands of seamen employed in the whale-fishery and other pursuits in the Pacific.

--Edgar Allen Poe, The Narrative of Arthur Gordon Pym of Nantucket

For sailors in tropical oceans before the invention of refrigeration, keeping supplies of food was a serious issue. It was a permanent challenge to keep supplies fresh and edible, and indeed, much of the time stores failed at both. Under such conditions, the giant tortoises of the Galapagos islands and the Mascarenes and other islands in the Indian Ocean would have been seen as nothing short of miraculous. Tortoises could be captured easily and kept in the hold of a boat for extended periods without feeding, only slaughtered when they were actually required for eating. As a result, ships that were in a position to do so often took on tortoises in large number, and Charles Darwin apparently recorded single vessels taking up to 700 individuals at a time. By modern standards the idea of seven hundred starving tortoises crammed into a single hull seems unthinkably cruel, but doubtless the sailors who otherwise faced another six months of decomposing ship's biscuit saw things differently.

Unfortunately, such intense harvesting took an inevitable toll. Tortoise numbers declined rapidly, and many went extinct. Honneger (1981) lists three extinct species of tortoise from the Galapagos (including Geochelone abingdoni from Pinta island, which is technically not yet extinct but which only survives in the form of a single captive male) and at least six extinctions from the Seychelles and Mascarenes. Extinct populations on the Galapagos islands of Rabida and Santa Fe may have represented further undescribed species.

However, a paper published yesterday in the Proceedings of the National Academy of Sciences adds a remarkable coda to the history of one of the "extinct" species, the Floreana tortoise Geochelone elephantopus. Using DNA extracted from museum specimens collected on Floreana before the population disappeared, Poulakakis et al. (2008) have demonstrated that G. elephantopus may not be quite as extinct as previously thought. Instead, anomalous genetic haplotypes previously identified in some living individuals of Geochelone becki, a species found on the Volcano Wolf at the northern end of Isabela, the largest island in the Galapagos, indicate descent from G. elephantopus. These individuals would appear to be descendants of past hybridisations between native Volcano Wolf tortoises and introduced Floreana tortoises.

Such a situation is quite believable. As a result of the widespread transport of tortoises for food, many tortoises ended up on islands to which they were not native*. Tortoises were regularly imported to Réunion in the Mascarenes after the native population became extinct. Living populations of giant tortoises on the Granitic Islands of the Seychelles probably descend from imports from Aldabra rather than representing the species originally found there (Honegger, 1981). According to Poulakakis et al. (2008), some 40% of the Volcano Wolf tortoises tested showed evidence of Floreana ancestry, so the genetic legacy of Geochelone elephantopus is alive and well, at least in hybrid form.

*Potentially a serious issue for taxonomy, as researchers cannot assume that species names based on inadequate type material necessarily represent the species native to the island the type was collected on. Honegger (1981), for instance, cast doubt on whether Geochelone gouffei, known from a single specimen found on Farquhar Island in the Seychelles, actually originated there.

This still leaves a significant problem - most conservation policies do not cope well with hybrids. A number of species worldwide, such as the black stilt (Himantopus novaezelandiae) in New Zealand, are regarded as endangered because of the risk of hybridisation with related species. The red wolf (Canis rufus) and the Florida panther (Puma concolor coryi) represent two 'endangered' taxa in the United States for which the suggestion that their histories could have been compromised by hybridisation led to the suggestion that they should be abandoned as worthwhile conservation targets. However, the disappearance or decline of a species in its pure form due to hybridisation with another species is a different proposition from its decline due to replacement by that species. The genetic legacy of the declining species may still persist. Overemphasis on species "purity" may actually hinder the conservation of endangered taxa, especially if natural hybrid zones with related taxa exist in the first place (Allendorf et al., 2001). If there are no purebred Florida panthers, should that mean that there is no place for panthers in Florida?

6 comments:

Wait, turtles are procolophonians again? Damn it. When did this happen!? Why doesn't anyone tell me about these things?

Anyway, you raise many interesting points as usual. The rediscovery of the Floreana tortoise is awesome. Still, the conservation recommendations in the paper--find and remove all of the Floreana descendants and restore the racial purity of both stocks--sounded a little too "eugenic" for my tastes.

I mean we can all kind of laugh at the whole "Pleistocene Park" proposal to "re"introduce lions and elephants to the plains of North America. Then again, no one really seems to get too bothered by the fact that U.S. peregrines are now heavily introgressed with European genes thanks to the captive breeding programs that restored their populations.

Those are opposite cases, but where is the line between them? If the Floreana tortoises are contributing some genetic diversity to the Isabela population is that really a problem that we need to fix? Does it really make sense to dump a lot of money and time (I mean were talking tortoises here) into a program to restore a pure (and presumably fairly inbred) population of Floreanas?

Wait, turtles are procolophonians again? Damn it. When did this happen!? Why doesn't anyone tell me about these things?

It didn't happen really. No-one knows what turtles are, and there was a certain tongue-in-cheek aspect to my listing them as "Procolophonia". You wait until I write a post on cephalopods and list them as "yochelcionelloids".

Nice blog you've got here. Thanks for linking to the Natural Patriot in your blogroll, and keep fighting the good fight, unabashedly proclaiming the wonder of obscure (and not so obscure) and beautiful (and less so) creatures.

And, yes, it is 8:20 PM on a Friday evening, which shows what a geek I am. Now back to real life . .

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The Author

I'm an entomologist and taxonomist, currently based in Perth, Western Australia. If you'd like to comment (or offer work), I can be e-mailed at gerarus at westnet.com.au.Subscribe to Catalogue of Organisms